Friday, September 3, 2010

In this post, I continue looking at the simple whole house embodied carbon emission model I studied here for a conventional home, and here for a straw bale home. Today I'm looking at what happens if we use SIPs - structural insulated panels - as the wall system. SIPs are large structural panels consisting of two skins of oriented strand board, plywood, or similar, bonded to either side of a thick layer of rigid foam (polystyrene, polyurethane or similar). SIPs are probably the leading technology for building super-insulated houses, and there are dozens of manufacturors in the United States producing them.

In particular, I assume that to get thermal performance comparable to a straw bale building we need to use a 10" SIP, containing about 8" of rigid insulation, giving about an R-40 wall once the insulation has aged a decade or so.

(Also, I note that in response to this comment by Jim Camasto, I added 2" of rigid foam insulation under slab and outside basement walls to the model for all three houses. That added about half a tonne of embodied carbon emissions in each case).

Here are the embodied and sequestered carbon emissions for the SIP house:

Comparing this to the conventional home:

Basically, the SIPs make modest increases in both the embodied carbon emissions and the sequestered carbon (essentially because the extra skin of OSB contains more material than the studs in the conventional home, as well as the additional insulation).

Of course, if the houses is heated/cooled with carbon based energy, the emissions saved through the life of the home, versus the conventional home, will dwarf these embodied/sequestered differences.

Comparing again the straw bale home:

the main environmental advantage of straw bale from this perspective is the sequestration.

In particular, if the operational energy of the house is to come entirely from renewables, so there will be no operational carbon emissions, then straw bale offers the possibility of having the house carbon negative on a embodied basis as well as on an operational basis.

There are of course a number of other considerations:

The aesthetics of straw bale are quite different, with thick walls with slightly wavy surfaces, versus the more conventional rectilinear appearance of SIP walls.

SIPS are much more widely available and familiar to architects, contractors, banks, and building officials - straw bale is still a bit fringe and is often done with significant amounts of owner labor. A SIP building is probably lower risk.

SIPs probably present larger issues with offgassing of toxics into the interior space.

Well, to my mind, counting the carbon in the straw/lumber is fair game, since it was actually pulled out of the air when we grew the crop/forest, and thus is helping incrementally to mitigate the CO(2) level in the air if we now fix it in the building. However, the carbon in the plastics was sitting harmlessly in the ground and there's no climate benefit in pulling it up out of there and putting it in a building.

I was thinking that as I wrote. But there is a limited amount of oil in the ground and it will probably all be pulled out eventually (more or less). Surely anything that is converted to plastics is not being burned so can be counted as sequestered?

Admittedly this is slightly like the plan we came up with while working on zerocarbonbritain - burying coal as a new type of carbon offset scheme. Where would you bury it all I hear you ask? Well there's these long tunnels deep underground that used to be full of...

BTW - in case anyone is following this over the weekend - I'm trying to come up with alternatives to poured concrete for a full basement that would have lower embodied energy, and I'm not coming up with much that's gotten a lot of testing, except concrete blocks, and concrete block foundations seem to fail a lot (the rental we are in at the moment has a lot of moisture incursion through the blocks that requires a dehumidifier which pulls a gallon a day out of the air).

Does anyone know of successful uses of, eg, lightweight/cellular concretes in basement applications (presumably in thicker walls to compensate for the lower strength)?

How about fiber-reinforced concrete? Fiberglas or (better yet) carbon fibers.

Actually, I think we should try to develop a way to make carbon buckytubes out of atmospheric CO2, then use the buckytubes to build a space elevator, and move a couple billion humans and all of our energy-intensive heavy industy off-planet, and turn Mother Earth into a huge nature preserve...

I would like to suggest another building approach that has low construction cost, is of conventional design so no permitting problems and is near heating and cooling neutral in a 7500 heating degree day, high wind area.

Find a south facing sloped lot and pour a three sided foundation. Pour with insulated forms or bury 3” foam against outside conventional concrete walls. Use truss roof construction directly on top of the concrete walls. You can form a slight internal vault with scissor trusses. It is less claustrophobic in big open rooms compared with a flat ceiling.

Some folks think dirt roofs make sense – I don’t. Use R60 roof insulation. Stick frame the south facing fourth wall with lots of high quality glass. A steel roof is probably preferred. Who knows if asphalt shingles will be available in 20 years?

Stud the internal walls and add an additional R15. Bury radiant tubing in the floor. Make sure you have the 3” foam under the floor and pay very close attention to drainage on a sloped lot. There are many more details but I think this picture is adequate for your consideration.

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About Me

I'm a scientist and innovator in the technology industry, with a broad range of interests and experiences. I have a Physics PhD, MS in CS, and have done research, lived in cohousing communities, run a business, and designed technology products. Professionally, I have mainly worked on computer security problems. Currently I'm Adjunct Professor of Computer Science at Cornell, but this blog represents my views only.
Email me at stuart -- at -- earlywarn -- dot -- org. I do read all email, but because the blog is a part-time unfunded enterprise, I often fail to reply due to lack of time - apologies.